1. Field of the Invention
The present invention relates to increasing the speed of a peak detector while minimizing ripple on its output.
2. Description of the Related Art
FIG. 1A illustrates a typical peak detector 100. The RF differential signal VRF(+) and VRF(−), whose envelope must be detected, is connected to the gates of NMOS transistors 101 and 102. Transistors 101 and 102 are connected in parallel between a voltage source VDD and a node 103, which provides an output signal Vo. An NMOS transistor 105 and a capacitor 106 are connected in parallel between node 103 and a voltage source VSS. In a separate path, a current source 107 and an NMOS transistor 104 are connected in series between voltage sources VDD and VSS. Current source 107 provides a bias current IB. The drain and gate of NMOS transistor 104 are connected to the gate of NMOS transistor 105, thereby ensuring that NMOS transistor 105 is also on and the bias current IB is provided at node 103.
In this configuration, when the input peak voltage exceeds Vo+Vt, where Vt is the threshold voltage of transistors 101 and 102, NMOS transistors 101 and 102 turn on and current flows through those transistors, thereby charging capacitor 106 and increasing the output signal Vo. In contrast, when the input peak voltage is equal to or less than Vo|Vt, NMOS transistors 101 and 102 turn off and capacitor 106 discharges. As a result, the output signal Vo decreases due to the constant leakage caused by bias current IB through NMOS transistor 105. Note that both NMOS transistors 104 and 105 are constant current devices that always carry bias current IB.
FIGS. 1B and 1C respectively illustrate exemplary graphs of VRF(+) and VRF(−) over time. FIG. 1D illustrates a graph of an envelope 110 of the differential signal, i.e. VRF(+) and VRF(−), as measured by the output signal Vo. As shown in FIGS. 1B, 1C, and 1D, the peak amplitude of each current pulse in Vo is proportional to the peak amplitude of the differential signal during that particular conducting half cycle. Thus, the peak values of the output current pulses follow the amplitude of the differential signal during conducting half cycles and have the same waveform as envelope 110. Note that in steady state, when the amplitude of the RF differential signal is constant, NMOS transistors 101 and 102 turn on for a small part of the period, which is enough to provide the charge that is removed by bias current IB. Unfortunately, this periodic charging of capacitor 106 can result in a ripple 111 (FIG. 1D) on the output signal Vo, which is generally undesirable.
In some applications, it is desirable to obtain a fast response from peak detector 100 when envelope 110 of the RF differential signal changes. For a given size of capacitor 106, a fast charge can be achieved by making NMOS transistors 101 and 102 large. A fast discharge can be achieved by making the bias current IB high. Unfortunately, when transistors 101 and 102 are large and the bias current IB is high, a large ripple appears on the output signal Vo.
Therefore, a need arises for a peak detector that can handle high bandwidth, i.e. ensure fast detector response, while minimizing ripple on the output signal.